Tubulointerstitial fibrosis, a common endpoint outcome of a wide range of chronic kidney diseases (CKD) that progress to end-stage renal failure, is preceded by activation of the a-smooth muscle actin- positive myofibroblasts, the principal effector cells that are responsible for the over-production of extracellular matrix components. This resubmission of a competitive renewal application is a continuation of our long-term efforts to elucidate the origins, activation process and fate of renal myofibroblasts in renal fibrogenesis. Studies in previous project period of this application suggest that myofibroblasts may originate from two major sources: from interstitial fibroblasts via a phenotypic activation and from tubular epithelial cells via epithelial-mesenchymal transition (EMT). In this renewal application, we propose to investigate the molecular mechanism and signal pathways leading to EMT and fibroblast activation. The central hypotheses to be tested are that: 1) transcriptional inhibitors Id1 and Id3 promote tubular epithelial to mesenchymal transition (EMT), via inducing tubular epithelial dedifferentiation and via potentiating renal inflammation;2) tPA acts as a profibrotic cytokine that promotes the survival and proliferation of interstitial fibroblasts and their myofibroblastic activation. These hypotheses will be addressed by three specific aims at the whole animal, cellular and molecular levels, respectively. Aim 1 is designed to investigate the regulation and function of transcriptional inhibitor Id proteins in mediating tubular EMT and renal inflammation. Aim 2 is to investigate the role of tPA in interstitial myofibroblast activation and to dissect the signaling pathway leading to its action. Aim 3 is to investigate the role of tPA in fibroblast and myofibroblast survival and proliferation. These studies will provide fundamental and important insights into understanding the activation mechanisms of myofibroblasts from both tubular epithelial cells and interstitial fibroblasts. Resolution of these fundamental issues will not only provide mechanistic insights into the pathogenesis of chronic renal fibrosis, but also offers unique opportunities for designing rational strategies for the treatment of this devastating disease. PUBLIC HEALTH RELEVANCE It is estimated that up to 11% of the US adult population has some degree of chronic kidney disease (CKD), and delaying the progression of CKD is still unsolved problem. The studies proposed in this application promises to provide important insights into understanding the origins, activation and fate of renal matrix-producing myofibroblasts in the pathogenesis of CKD. Resolution of these fundamental issues may offer unique opportunities for designing rational strategies for the treatment of human CKD.